1. Field of the Invention
This invention relates generally to atomic frequency standards, and more particularly to evacuated, wall-coated, sealed alkali atom cells for rubidium (or Cesium) atomic frequency standards.
2. Description of the Prior Art
Commercial rubidium frequency standards use cells filled with buffer gas. A primary function of such a gas is to slow Rb atom diffusion to the wall, thereby providing a longer lifetime for the hyperfine states being interrogated. Systematic effects associated with this choice include temperature and pressure coefficients of the hyperfine-structure (hfs) transition frequency, and an inhomogeneously broadened line shape which results in a shift of the line center with light intensity, cell temperature, rf power, rf field gradients, and magnetic field gradients.
An alternative containment device proposed by Ramsey Rev. Sci. Instrum. 28 57 (1957), and first demonstrated in a Rb optical pumping experiment Rev. Sci. Instrum. 28 57 (1957) by Robinson, Ensberg, and Dehmelt Bull. Am. Phys. Soc. 3, 9 (1958), is the wall-coated evacuated cell.
Robinson and Johnson, Appl. Phys. Letl. 40 (90), May 1, 1982, with interest toward exploring the potential of such cells for use in frequency standards, presented results on the line shape of .sup.87 Rb hfs resonances showing significant improvement over previous results. In particular, a conventional optical pumping apparatus employing a Rb resonance lamp, D1 pass filter, circular polarizer, cell, and Si photodetector was used. A magnetic field of 1.5 G was applied to resolve all transitions. The rf power for observation of the hfs transitions was provided by a waveguide horn or a loop antenna driven by a klystron phase locked to a harmonic of a frequency synthesizer. A low modulation rate of 0.3125 Hz was used since the response time of the Rb atom ensemble in the cell was about 1 s. The analog output of the Si photodetector was amplified and demodulated by means of a phase-sensitive detector. This signal was then digitized by a voltage-to-frequency converter and a gated counter. A computer interfaced not only to this counter but also to the frequency-synthesizer controlled data acquisition.
Preparation of the 200-cm.sup.3 spherical Pyrex cell included the formation of wall orifices together with ring-sealed tubing centered on each orifice. Typically, these are present for primary evacuation, for introduction of the wall coating, for connection to the .sup.87 Rb reservoir (98% isotopic purity), and for future breakseal access to the cell, if desired. Cell cleaning included a 12-h vacuum bakeout at 350.degree. C. followed by plasma scrubbing with both hydrogen and helium gases. Purified n-tetracontane was evaporated onto the wall from a removable central wax carrier and the Rb reservoir ampoule attached to the cell was then opened. Surface coating and curing were done under active vacuum pumping. Since the cell used had been sealed off from any pumping since 1968 (over ten years), it was apparent that long-term operation of such cells is feasible. Robinson and Johnson, supra, therefore monitored the time dependence of cell parameters to ascertain their stability.
The experiments performed by Robinson and Johnson, supra, demonstrated that an evacuated wall-coated cell which had been sealed for more than 10 years can have hfs resonances whose linewidths are a factor of 70 less than those in conventional buffer gas filled cells used in modern Rb atomic frequency standards. Robinson and Johnson, supra, theorized that since the Q of the resonance is increased by this same factor, the flicker-of-frequency noise floor (H. Hellwig, Radio Sci., 14, 561 (1979)), would be substantially improved, and that because the line shape in the wall-coated cell is homogeneously broadened, the major systematic effects limiting the precision of current Rb atomic frequency standards should be greatly reduced. Robinson and Johnson, supra, concluded that although questions of reproducibility and stability of the wall shift remain to be explored, the achievement of so narrow a linewidth in a long term sealed cell showed potential for use of the wall-coated cell in frequency standards.
Examples of gas cell atomic frequency standards are disclosed in U.S. Pat. Nos. 4,798,565 and 3,903,481.
Other prior art publications of interest are Risley et al, J. Appl. Phys. 51, (9), Sept., 1980; Brewer, J. Chem. Physics., Vol. 38, Number 12, June 15, 1983, pp. 3015-3020, and Bouchiat et al, Physical Review, Vol. 147, No. 1, July 8, 1966, pp. 41-57.
The disclosure of each of the above-noted publications, as well as the other prior art noted therein, are incorporated by reference herein.